热门搜索：如何备考GRE GRE填空 GRE数学机经 GRE模考
In prehistoric times brachiopods (brachiopod: n.腕足类动物的一种any of a phylum (Brachiopoda) of marine invertebrates with bivalve shells within which is a pair of arms bearing tentacles by which a current of water is made to bring microscopic food to the mouth) were one of the most abundant and diverse forms of life on Earth: more than 30,000 species of this clamlike creature have been cataloged from fossil records. Today brachiopods are not as numerous, and existing species are not well studied, partly because neither the animal’s fleshy inner tissue nor its shell has any commercial value. Moreover, in contrast to the greater diversity of the extinct species, the approximately 300 known surviving species are relatively uniform in appearance. Many zoologists have interpreted this as a sign that the animal has been unable to compete successfully with other marine organisms in the evolutionary struggle.
Several things, however, suggest that the conventional view needs revising. For example, the genus Lingula has an unbroken fossil record extending over more than half a billion years to the present. Thus, if longevity is any measure, brachiopods are the most successful organisms extant. Further, recent studies suggest that diversity among species is a less important measure of evolutionary success than is the ability to withstand environmental change, such as when a layer of clay replaces sand on the ocean bottom. The relatively greater uniformity among the existing brachiopod species may offer greater protection from environmental change and hence may reflect highly successful adaptive behavior.
The adaptive advantages of uniformity for brachiopods can be seen by considering specialization, a process that occurs as a result of prolonged colonization of a uniform substrate. Those that can survive on many surfaces are called generalists, while those that can survive on a limited range of substrates are called specialists. One specialist species, for example, has valves weighted at the base, a characteristic that assures that the organism is properly positioned for feeding in mud and similar substrates; other species secrete glue allowing them to survive on the face of underwater cliffs. The fossil record demonstrates that most brachiopod lineages have followed a trend toward increased specialization. However, during periods of environmental instability, when a particular substrate to which a specialist species has adapted is no longer available, the species quickly dies out. Generalists, on the other hand, are not dependent on a particular substrate, and are thus less vulnerable to environmental change. One study of the fossil record revealed a mass extinction of brachiopods following a change in sedimentation from chalk (a soft white, gray, or buff limestone composed chiefly of the shells of foraminifers) to clay. Of the 35 brachiopod species found in the chalk, only 6 survived in the clay, all of them generalists.
As long as enough generalist species are maintained, and studies of arctic and subarctic seas suggest that generalists are often dominant members of the marine communities there, it seems unlikely that the phylum is close to extinction.
当前版本由长颈鹿在哪里呀更新于2019-02-11 17:29:43 感谢由长颈鹿在哪里呀对此题目的解答所做出的贡献。
这道题可以直接定位到Today brachiopods are not as numerous, and existing species are not well studied,partly because neither the animal’s fleshy inner tissue nor its shell has any commercial value，可表明没有社会和商业价值是其中的影响因素。
[阅读&逻辑 ]经典真题320篇 -9340
The highlighted sentence serves primarily to
[阅读&逻辑 ]经典真题320篇 -9339
The author would most likely agree with which of the following assertions about the environmental impact of hydroelectric power plants?
[阅读&逻辑 ]经典真题320篇 -9338
The passage suggests that which of the following contributed to the erosion after the First World War of the progress previously realized by the labor movement in the United States?
[阅读&逻辑 ]经典真题320篇 -9337
According to the passage, which of the following statements about the National War Labor Board is true?
[阅读&逻辑 ]经典真题320篇 -9336
It can be inferred from the passage that which of the following was one of Local 36’s grievances against the Little Rock laundry owners?
[阅读&逻辑 ]经典真题320篇 -9335
The passage is primarily concerned with discussing
[阅读&逻辑 ]经典真题320篇 -9334
According to the passage, Appalachian square dancing differs from English country dancing in that Appalachian square dancing
[阅读&逻辑 ]经典真题320篇 -9333
The passage suggests that which of the following was true of
[阅读&逻辑 ]经典真题320篇 -9332
The statements given are structured to lead to which of the following conclusions?
[阅读&逻辑 ]经典真题320篇 -9331
Consider each of the choices separately and select all that apply. It can be inferred that historical demographers generally believe which of the following about communities of fewer than 10,000 people in early-modern Germany?
[阅读&逻辑 ]经典真题320篇 -9330
The author suggests that according to historical demographers, the birth rate in early-modem German cities
[阅读&逻辑 ]经典真题320篇 -9329
Which of the following most logically completes the argument?
[阅读&逻辑 ]经典真题320篇 -9328
The highlighted portion of the passage serves primarily to
[阅读&逻辑 ]经典真题320篇 -9327
According to the passage, certain African literatures have
[阅读&逻辑 ]经典真题320篇 -9326
It can be inferred from the passage that the Tea Cess Committee reorganized as the Indian Tea Market Expansion Board in order to
[阅读&逻辑 ]经典真题320篇 -9325
The passage suggests that "small packets of leaf fragments" were
[阅读&逻辑 ]经典真题320篇 -9324
In the argument in the editorial, the part that is highlighted plays which of the following roles?
[阅读&逻辑 ]经典真题320篇 -9323
Consider each of the choices separately and select all that apply.Which of the following can be inferred from the passage about the"period of relative warmth"
[阅读&逻辑 ]经典真题320篇 -9322
Which of the following best describes the organization of the passage?
[阅读&逻辑 ]经典真题320篇 -9321
It can be inferred that the author would agree with which of the following statements about glaciers?